[sockshare] Add new extractor

Hi,
this adds an `InfoExtractor` for sockshare.com, also addressing issue #3216.

The test video is [Big Buck Bunny](http://www.bigbuckbunny.org/), which is [licensed](http://www.bigbuckbunny.org/index.php/about/) under [Creative Commons Attribution 3.0](http://creativecommons.org/licenses/by/3.0/).

Suggestions and improvements are very appreciated.